Improving Accuracy and Efficiency of Aerodynamic Simulations for Heavy Vehicles

Examensarbete för masterexamen
Johansson, Simon
Heavy-duty vehicles are a necessity in the current society but they answer to 6% of the total CO2 emissions in Europe. Therefore new regulations are set by the European Union to limit those emissions. These limits make the losses of the vehicles an important factor and especially aerodynamic drag. Reducing the aerodynamic drag calls for good developing methods and accurate predictions. This means that validations and limitations of the methods are needed. This thesis will investigate a CFD method by creating a base case simulation and validate this against wind tunnel test. The method will use unsteady simulations to capture transient behaviour in the flow. A statistical confidence intervals for the averaged values as well as a starting point for the averaging will be presented. Further, the spatial mesh will be studied and the effects of the number of inner iterations will be presented. The validation of the base case will be stated as differences in the coefficients Cd , Cs , and Cp to understand advantages and disadvantages of the method. The frequency content from the time-resolved coefficients will also be studied. When a well-defined base setup has been validated the temporal resolution will be investigated to see how it affects the results. This will be done for increased time steps as well as a significantly reduced time step. The significantly reduced time step ensures the CFL number to be less than unity in the entire domain. The validation of the base case shows that there is an error in Cd of 45 ± 7 drag counts for the base case and that the error in pressure on the rear end of the truck is yaw angle dependent. The error in Cd is almost fully explained by the error in pressure on the rear end of the truck. The result from the simulations with different time steps show that for a fully resolved flow very fine time steps are needed to keep the CFL number less than unity in the majority of the domain, but if global force coefficients are of primary interest the time step can be increased moderately without significant changes. It is also found that the force coefficients alone are not a good estimators of how the method performs since errors can cancel each other. This means that a coarser mesh or a longer time step can generate a smaller error compared to wind tunnel tests but this is not reflecting the actual accuracy of the method.
Heavy-duty vehicles, Aerodynamics, CFD, Wind tunnel correlation, Absolute accuracy, Temporal resolution
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